THE ROOT. 



75 



the root, that is, they are 

 tangential divisions. 



The outer growing layer 

 or cork cambium is in the 

 great majority of plants 

 formed from the cells of the 

 pericycle, but it may be 

 produced by some of the 

 cells of the cortex. In any 

 case the tissues which arise 

 from this division are of 

 such a nature as to protect 

 the parts within. They con- 

 stitute the, periderm (fig. 87), 



Fig. 88. — Transverse section of two bundles 

 from the periphery of the stele of root of broad 

 bean (Vicia Fab a) at the beginning of secon- 

 dary thickening. The xylem bundle, g, is 

 shaded; the phloem bundle unshaded, v, the 

 stelar cambium; /, the pericycle, aiso showing 

 tangential divisions in parts ; s, the endoder- 

 mis. Highly magnified.— After Haberlandt. 



Fig. 89.— Transverse section of the stele of root of bean {Phaseolus multiflorus) shortly 

 after secondary thickening has begun, .r, endodermis; />c, pericycle; b, phloem bundles; 

 /, primary xylem bundles; g,g', secondary xylem; c, stelar cambium; M, central pith. 

 Compare with fig. go. Highly magnified.— After Sachs. 



